Process instruments such as current-to-pressure transducers and valve positioners are conventionally employed in industrial processes and are controlled by a process controller which is generally located remotely from and coupled to the transducer or positioner by a two-wire link. The process controller provides a variable DC control current signal of between 4 and 20 mA over the two-wire link to the transducer or positioner or to any other controllable device or instrument. The control current level changes the state of the controllable device in proportion to the strength of the variable DC current signal. For example, a valve positioner might fully open a valve in response to a 4 mA control current and fully close the valve in response to a 20 mA control current.
In addition to being responsive to a variable control signal, current-to-pressure transducers and valve positioners have variable parameters which may be adjusted to control the operating characteristics of such devices. Previously, these devices or process instruments were adjusted manually when it was necessary to change the instrument parameters such as the zero level, instrument range, output pressure, or valve travel.
With the advent of so-called "smart" devices capable of bidirectional communication, it has become possible for the above-described adjustments to be made automatically and from a location remote from the device or field instrument. Moreover, diagnostic testing and instrument monitoring can also be conducted from a remote location. However, means must then be provided for transmitting a communication signal from a communication site to the field instrument in order to implement the adjustments and the field testing.
Because the process controller and the communication site are often located a substantial distance from the field instrument, undue effort and expense is required to provide communication lines independent from the two-wire control loop interconnecting the communication site with the field instrument. Accordingly, it is desirable to transmit the communication signal over the two-wire control loop together with the 4-20 mA control signal so that additional wiring will not be required. To that end, the modulated digital communication signal is superimposed on the 4-20 mA DC analog control signal used to control the field instrument in order to allow serial communication of data bit streams between the field instrument and the communication site.
However, the presence of the modulated digital communication signals, i.e., the AC communication signals on the two-wire control loop can adversely affect the performance of the process control system, and undesirable characteristics of the process controller can adversely affect the AC communication signals. For example, some process controllers have an output impedance that is low enough to attenuate the communication signal so that the communication signal cannot be reliably detected by the field instrument. As another example, some process controllers output a 4-20 mA DC analog control signal containing transients, noise, or other undesired AC components that can interfere with the modulated digital communication signals on the two-wire control loop. As a third example, during digital communication, the modulation of voltage on the two-wire control loop can adversely affect the output current of the process controller or current readback circuits.
It is therefore desirable to couple an interface apparatus between the process controller and the field instrument. Such an interface apparatus must be designed to regulate the analog control signal to remove unwanted noise therefrom and to effectively isolate the process controller from the digital communication signal by providing the interface apparatus with an output having a high impedance to frequencies present in the digital communication signal.
Several attempts to integrate such an interface apparatus into a process control loop have proven unsuccessful. A passive RC circuit, for example, either has an output impedance that is too low to allow modulation of the voltage by the communication signal or has an input-to-output head voltage that is too high to maintain electrical compatibility between the process controller output signal and the device. In limited circumstances, an inductor could achieve some of the benefits of the present invention, but, the inductor would have to be prohibitively large and heavy in order to achieve those benefits.